Will Hot Weather Kill the Coronavirus Where You Live?

For many people living with the crushing consequences of Covid-19, the summer offers a tantalizing possibility: If the coronavirus behaves like the seasonal flu, warm weather could substantially weaken the virus and allow normal life to resume. President Trump predicted exactly this outcome in February, claiming the virus would “miraculously” go away by April as temperatures rose.

A new working paper tries to put that speculation to rest by tracking how weather and other environmental conditions, such as pollution, affect the virus’s spread around the world.

The forecast from researchers is grim: Warm weather alone will not control the virus in America or abroad. Here are the results for the United States, showing weather on its own cannot meaningfully reduce infections to the rate of 1 new case per every infected person, the point by which the number of infections falls continuously.

Shows metropolitan areas and cities with more than 500,000 residents. Note: Assumes a reproductive number of 2.6, meaning one infected person will, on average, infect 2.6 other people. This value can vary from place to place and does not take into account the effect of actions like social distancing nor a rise in the population that’s immune to the virus.

Without social distancing and other interventions, summer will offer only a modest respite in some places, meaning stay-at-home orders and other government interventions will most likely need to continue throughout the summer.

“At the end of the day, this whole effect from weather and pollution is still pretty minor,” said Mohammad Jalali, an assistant professor at Harvard Medical School and one of the study’s authors. “No government should rely on the effect of the weather.”

The study, conducted by researchers at six academic institutions, found that warm weather could play a small role in slowing the virus in at least a few places and for a few months. In some of the hottest cities in the United States, like Phoenix, high temperatures could drive down the rate of infections by over 40 percent. In parts of India and Pakistan, conditions during the hotter months could make the virus less than half as likely to infect new hosts.

For regions facing cold, bitter winters, weather has the opposite effect, increasing the rate of infections and making it even harder to control the virus.

Like all weather forecasts, there is significant uncertainty about the results. Researchers will continue to debate the measurements and methodology; others may come up with alternative projections. But all the evidence available suggests weather plays a role with coronavirus transmission, but only a small one.

Here is how those predictions look in every U.S. county for the next year.

1.6

3.6

New cases per infected person

Warmer and

less infectious

Colder and

more infectious

1.6

3.6

New cases per infected person

Warmer and less infectious

Colder and more infectious

Weekly average based on 2019. Note: Assumes a reproductive number of 2.6, meaning one infected person will, on average, infect 2.6 other people. This value can vary from place to place and does not take into account the effect of actions like social distancing nor a rise in the population that’s immune to the virus.

In New York City, weather could reduce the rate of infections by a quarter during the hottest months. But by winter, those benefits likewise fade, when cold weather could boost transmissions slightly.

Headed into summer, governors have already signaled their eagerness to give up social distancing and reopen their economies before passing the White House’s own guidelines for testing and new infections. The warm weather effects could help justify those decisions in the short term, as governors point to falling infections as a sign that the virus is defeated.

“Basing policy decisions on the assumption of seasonality is dangerous,” said Anice Lowen, a professor of microbiology and immunology at Emory University, which was not affiliated with this study. “Possibly we’ll see slower spread in the summer, but even that summer spread could seed the virus in a quiet way, and then come winter it would be a bigger problem than it would have been.”

There are some silver linings, however. Rather than relying on weather alone to slow the spread, it’s possible that warm temperatures will work alongside other interventions to adequately slow the virus. Here’s how that works:

This is the forecast for several major U.S. cities without any human intervention. Each infection spawns two to three new infections. Even the most favorable weather conditions don’t slow the spread of the virus enough.

Here is the same forecast but with human interventions in place. Transmissions are curbed by at least 40% across all parts of the country. The weather effect helps push down the infection rate below the critical threshold where infections will decrease, but only in the hottest parts of the country.

If every state pushes to decrease transmission by 60%, the entire country will see a reprieve during the summer. But in most places, even that level of intervention will allow infections to rise again by fall and winter.

It’s not fully understood how warm weather controls the spread of the virus.

Akiko Iwasaki, an immunologist at Yale University who wasn’t affiliated with this study, said that people are more susceptible to infections in other respiratory viruses when the air they breathe is cold or particularly dry.

Research has also shown that the hotter it is, the faster some viruses will break down outside of a human host. Scientists have found evidence that this coronavirus dies in warmer weather as well. And dry air helps respiratory viruses in droplets expelled from a person’s nose or mouth stay in the air longer, making them more likely to infect someone else.

Some differences may be driven by how people react to the weather. Regions with more ultraviolet light, like Colombia and Ecuador, were associated with higher transmission rates in the study, even though it’s believed that ultraviolet light can destroy the virus. Hazhir Rahmandad, a co-author of the study from the Massachusetts Institute of Technology, suggested a high UV index could drive people indoors, where it’s easier to spread the virus.

“It’s a combination of host susceptibility and viral stability,” Dr. Iwasaki said. “In dry and colder conditions the virus survives better, combined with the fact that host resistance in those conditions is impaired.”

As it becomes harder for the virus to find new people to infect, it’s likely that Covid-19 will ebb and flow with the seasons, according to Dr. Lowen, the Emory microbiology professor. But for now, too many are susceptible to the virus for the weather to influence when and where infections will rise and fall.

What has undoubtedly influenced the virus’s spread, however, is the human factor: decisions made by governments and individuals alike. While the weather effects are slight, some countries have successfully controlled the virus.

The weather can’t be controlled. But our actions can.

Nathaniel Lash is a graphics reporter. Stuart A. Thompson is a writer and the graphics director for Opinion.

MethodologyTo calculate the weather effects, researchers compared changes in coronavirus infections with data on temperature, humidity, precipitation, air pressure, wind speed, UV index and pollution in different climates around the world. After validating their statistical method using an epidemic simulation model, they used the statistical model to forecast what would happen as weather changed, based on historical weather patterns.

To know how weather will ultimately influence the spread of the virus, researchers need to know how easily it is spreading in each region, while adjusting for changes in behavior and policy that also influence the rate of infections. That figure depends on how many people are susceptible to the virus and how much contact people have with one another, in addition to the environmental effects that were measured in the study. Charts in this story presume a basic reproductive number of 2.6, meaning one infected person will on average infect 2.6 other people, a common estimate based on studies from different countries. The charts also assume that everyone is susceptible to infection. Weather conditions by region are based on the previous year's weather, and thus likely to vary from these projections.